Demodex mites (of the class Arachnid and order Acarina) are microscopic ectoparasites that commonly infest the pilosebaceous unit of the skin. Among a wide range of reported species, at least Demodex folliculorum (D. folliculorum) and Demodex brevis (D. brevis) are found on the human body surface. Both Demodex species often coexist and preferentially gather at the same skin area of the face, cheeks, forehead, nose, and external ear tract, where active sebum excretion favors their habitat and breeding. Adult Demodex mites eventually die after a life cycle of 14 to 18 days, and their expansion is dependent on successful copulation by adult mites. Copulation takes place in the opening of the hair follicle (near the skin surface). Afterwards, the gravid female moves to the sebaceous gland to deposit eggs, which give rise to larvae and then protonymphs. A protonymph is brought to the opening of the hair follicle and gives rise to a deutonymph, which crawls onto the skin surface and re-enters a hair follicle to become an adult. Therefore, during a life cycle, if adult Demodex can successfully copulate and produce the next generation, the extent of infestation will increase in the host.
Demodex infestation is non-existent in healthy children, increases in an age-dependent manner, and is found probably 100% in elderly skin, unless eradicating measures are taken. The prevalence of Demodex infestation is higher in unhealthy skin than in normal skin. Overgrowth of these mites is linked to blepharitis (an inflammation of the eyelids), and other inflammation-associated conjunctival and corneal abnormalities. Blepharitis occurs as an ulcerous (staph) or nonulcerous (seborrheic) form, or a combination of both. Patients who suffer from blepharitis as a result of an ocular Demodex infestation often present a number of symptoms such as foring body sensation, redness and itching. Uncontrolled ocular Demodex infestation in eyelids may cause mal-directed lashes (trichiasis), meibomian gland dysfunction leading to lipid tear deficient dry eye, conjunctival inflammation (conjunctivitis), and sight-threatening corneal abnormalities. The symptoms can become severe enough that the patient may require surgery to achieve relief.
Rosacea is a chronic dermatological disease that affects the skin, usually the face, and sometimes the eyes. Inflammatory rosacea causes persistent redness and pink bumps referred to as papules, and pustules on the skin. Eye inflammation also may occur, with symptoms often including sensitivity to light, blurred or otherwise impaired vision, redness, dryness, itching, burning, tearing, and the sensation of having grit or sand in the eye. Inflammation of the eye is more apparent in advanced stages of rosacea, where the skin thickens and becomes a deep shade of red. Current treatments include oral antibiotics, e.g., tetracycline or doxycycline. If infections of the eyelids develop, physicians may recommend scrubbing the eyelids with diluted baby shampoo. Steroid eye drops may be prescribed in the case of severe infection.
Acne, including acne vulgaris and acne rosacea, is yet another chronic dermatological condition that is difficult to treat. Over-the-counter products for treatment of acne, including benzoyl peroxide and aluminium chlorhydroxide/sulphur can help reduce, but not cure, acne. Prescription treatments, including antibiotics, retinoids, and certain hormone pills, can improve acne, but can have serious adverse effects.
Demodex can also infest mammalian quadrupeds, in particular domestic animals, especially dogs, causing demodetic mange. The Demodex mite burrows into hair follicles and sebaceous glands of the animal, often causing severe dermatitis, infection, and discomfort. Demodetic mange, particularly in dogs, presents a difficult clinical problem for veterinarians, as it can involve the face and the entire body of the animal in some cases. Existing treatments can be expensive and are not always effective, with the result that affected animals are sometimes euthanized. Mange can also be caused by the burrowing parasitic mite Sarcoptes, which causes scabies, and Chorioptes. The Cheyletiella mite causes a condition known as “walking dandruff.” In cats, notoedric mange is a burrowing mite infestation that is difficult to treat.